1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device, and more particularly, to an organic EL display device having an organic soluble derivative layer.
2. Description of the Related Art
An organic EL display device includes an anode, a hole injection layer, a hole transporting layer, a light-emitting layer, an electron transporting layer, an electron injection layer, and a cathode which are sequentially stacked on a substrate. In the case of an organic EL display device comprising a low-molecular material, the above-described layers are formed using a vacuum deposition technique. In the case of an organic EL display device comprising a polymer, the above-described layers are formed using a spin-coating technique, which places limitations on forming the light-emitting layer thereof.
FIG. 1 shows a cross-sectional view of a conventional organic EL display device comprising a high-molecular material, such as a polymer. The organic EL display device includes an anode 10, a hole injection layer 12, a light-emitting layer 14, and a cathode 16 which are stacked in sequence as shown.
The hole injection layer 12 is formed using a spin coating technique, and has a structure such that the hole injection layer 12 is not dissolved in an organic solvent during a spin coating operation to form the light-emitting layer 14. As a result, an interface characteristic between the hole injection layer 12 and the light-emitting layer 14 deteriorates. In addition, water or inorganic and organic impurities affect the light-emitting layer 14, thereby lowering stability and shortening a life span of a resulting device.
U.S. Pat. No. 5,247,190 discloses an organic EL display device including an anode electrode and a cathode electrode with a light-emitting polymer interposed therebetween. The organic EL display device of U.S. Pat. No. 5,247,190 has a disadvantage in that a light-emitting efficiency is low and a life span is short because a work function of the two electrodes is not approximate or identical to a highest occupied molecular orbital (HOMO) value and a lowest unoccupied molecular orbital (LUMO) value of the light-emitting polymer.
In order to overcome the problem, the cathode electrode is made of a metal having a work function similar to the LUMO value of the light-emitting polymer. However, since the anode electrode has to be made of a transparent conductive material, there are many restrictions to using an anode electrode material.
Accordingly, a water-soluble material such as PEDOT (mixture of a poly(3,4)-ethylenedioxythiophene and a polystyrenesulfonate) or PANI (mixture of a polyaniline and a polystyrenesulfonate), available from Bayer AG and Allied Signal, respectively, is deposited on the anode electrode using a spin coating technique in order to form a hole injection layer. The hole injection layer buffers an interface between the anode electrode and the light-emitting polymer, thereby increasing a light-emitting efficiency, a driving voltage and a life span.
However, the PEDOT and the PANI are ionic materials which dissolve in water and are high in absorptiveness. Thus, even though a firing process is performed after the spin coating operation, it is almost impossible to remove the water remaining on a surface thereof. In particular, since the PEDOT and the PANI show a strong acidity when water exists therein, a reduction is likely to occur on an interface between the hole injection layer and the light-emitting polymer.
In addition, the PEDOT and the PANI provide poor adhesion to the light-emitting polymer. Therefore, where the light-emitting polymer is patterned to form a light-emitting layer, the light-emitting layer having a bad pattern characteristic is formed.
Currently, much research is being conducted to improve an interface characteristic between a hole injection layer and a light-emitting layer by introducing a surfactant or an adhesive.
On the other hand, U.S. Pat. No. 5,998,085 discloses a method of forming R, G and B light-emitting layers of an organic EL display device using a laser induced thermal imaging (LITI) technique. The LITI technique requires at least a light source, a transfer film and a substrate. Light emitted from the light source is absorbed by a light absorbing layer of the transfer film and is converted into a heat energy. An image forming material on the transfer film is transferred to the substrate by the heat energy to thereby form a desired image. The LITI technique is also used to form a color filter of a liquid crystal display (LCD) device and a light-emitting layer.
However, a conventional LITI technique shows a bad transfer characteristic. Therefore, there is a need for a material and a structure which improve a transfer characteristic of a light-emitting polymer of an organic EL display device.